1. Field of the Invention
This invention relates to a circuit for amplifying dynamic focussing parabolic signals for magnetically focussing a scanning electron beam in a cathode ray tube (CRT).
Of interest is commonly owned copending application Ser. No. 800,891 entitled Dynamic Focussing Circuit for Cathode Ray Tube and Transformer for Use Therein filed Nov. 27, 1991 in the name of Golik.
2. Description of the Prior Art
Dynamic focussing circuits are widely used in television receivers for improving the focussing attributes of the electron beam scanned across the face of a CRT producing the display. The CRTs in use may either be electrostatic or magnetically driven devices. In an electrostatic CRT, the dynamic focussing circuit is voltage driven, the focussing signal comprising combined horizontal and vertical parabolic voltages applied to an electrode inside the CRT. The combined voltages are of relatively low power, i.e., low current. In a magnetically focussed CRT, focussing coils are external the CRT in the yoke region and are current driven. These coils need relatively high power to generate appropriate magnetic focussing fields internal the CRT to perform their focussing operation. Because of the relatively high power requirements and the need for appropriately shaped waveforms for driving the horizontal and vertical magnet focussing coils, a separate power amplifying stage is provided for the horizontal and for the vertical focussing coils.
The above noted copending application relates to an electrostatic focussing circuit. In addition to this copending application, examples of different kinds of dynamic focussing circuits are illustrated in U.S. Pat. No. 4,546,291, a magnetic coil focussing arrangement and U.S. Pat. No. 4,485,335, British 908,755 and Japanese Patent Application No. 62-308072, the latter referring to electrostatic focussing circuits.
The beam deflection coils, without a dynamic focussing correction circuit, exhibit undesirable defocussing of the beam spot at the phosphors of the display face due to the different radii of the display face receiving the scanning beam and that of the beam. Thus a different portion of the beam is incident on the display face edges of as compared to the face center. In the magnetic system, a horizontal parabolic signal unique to the horizontal focussing coil is applied to the horizontal focussing coil to correct for defocussing of the beam in the horizontal direction. A different vertical parabolic signal unique to the vertical focussing coil is applied to the vertical focussing coil to correct for defocussing of the beam in the vertical direction of the scan. These parabolas are generated at different frequencies corresponding to the vertical and horizontal scan rates, for example, 31.475 KHz horizontal rate and 60 Hz vertical rate in the U.S.
In a magnetic focussing system, the dynamic focussing circuits include a power output stage for each of the horizontal and vertical parabolic signals. The shape of the waveforms and their voltage and power levels is important. For example, a relatively low voltage, e.g., 20 volts, may be used to drive the vertical focussing coil while a relatively high voltage, e.g., 120 volts, drives the horizontal focussing coil. Further, a parabolic waveform is used to drive the vertical focussing coil and a saw tooth voltage is used to drive the horizontal focussing coil. The focussing signals are parabolic voltages which need to be power amplified to the appropriate power and voltage levels as well as converted to the appropriate waveshape for the designated magnetic coil. This makes the power output stages relatively costly as well as taking up real estate in the appropriate circuit boards.
This complexity and cost is not a problem in general television receivers which, for example, use only two power output stages. However, under development are high definition projection television receivers. The latter employ three CRTs each dedicated to a different color channel, e.g., red, blue and green. Since each CRT requires a pair of power output stages, this kind of receiver with three CRTs requires six output stages. The present inventor recognizes a need to reduce the number of, and thus the cost of, the power output stages in such an environment.